Hybrid vehicles (HEVs) have propulsion systems that consist of at least one electric motor or electric machine in combination with at least one other power source. Typically, the other power source is a gasoline or diesel engine. There are various types of HEVs depending on how the electric motor(s) and other power source(s) are combined with one another in order to provide propulsion for the vehicle, including series, parallel and compound HEVs.
Powertrain architectures for HEVs manage the input and output torques of various prime movers, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to an energy storage system, comprising a battery pack. The internal combustion engine in a series HEV is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel HEV architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel HEV powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions (i.e., input-split, output-split and compound-split configurations) thereby enabling high-torque continuously variable speed ratios, electrical energy-dominated launches, regenerative braking, engine off idling, and two-mode operation.
As noted, such complex EVT HEVs utilize one or more electric machines and require advanced energy transmission, conversion and storage systems to supply electrical energy to and receive and store electrical energy from these machines, and typically comprise, for example, at least one electric machine, power inverter module, power bus, electrical energy storage device, such as a battery, as well as various control electronics, control algorithms and other associated items. The energy storage system (ESS) may comprise any suitable energy storage system that is adapted for high-density energy storage, including a battery, ultracapacitor, or other high-density energy storage device. As used herein, reference to a battery includes not only a single battery, also includes any combination of single or multiple batteries, or cells thereof, into a battery pack or array, or a plurality of battery packs or arrays. As used herein, the term battery generally refers to any secondary or rechargeable battery.
Significant attention has been given to maintaining the operational performance of batteries used in HEV applications, including maintaining the battery pack state of charge (SOC). The SOC is defined generally as the ratio of the residual charge in a battery relative to its full charge capacity. Various hardware and software control strategies have been adjusted for determining and maintaining the SOC of the battery.
Vehicles, including HEVs, are expected to accelerate in response to operator torque requests, including achieving various launch characteristics, e.g., an elapsed time to reach a speed. A vehicle launch is generally associated with starting the motion of the vehicle from a stop, typically characterized by the speed of the vehicle, such as from zero km/h to thirty km/h, and a required torque output. Launch conditions also exist during other periods of vehicle operation, such as acceleration from a low-speed interval, or seeking to maintain or increase speed while negotiating an incline.
A hybrid system application can underutilize the energy storage system, due to several factors including the size and power capacity of the primary power source, i.e., the internal combustion engine, and, the specific speed/load duty cycle of the vehicle. In at least one specific case the maximum energy storage usage has been shown to be about half of an allowable usage limit. In a hybrid system, it is desirable to make full use of the energy storage system in transient operating conditions, i.e., acceleration and decelerations, to reduce the fuel usage.
Current operating systems typically optimize fuel economy by minimizing the power losses associated with operation at a specific output torque and speed (thus a specific power). This is accomplished by solving equations at quasi-steady state operating points to direct power flows from the primary power source or the secondary power source.
Current system operation can be described with reference to an operator torque request (To_req) in the form of a throttle tip-in/tip-out maneuver. The operator torque request (To_req) is typically input to the system via the throttle, which is linked to an output torque command (To_cmd) in the hybrid control system. The hybrid control system monitors system operation at each operating point as the vehicle accelerates, and determines a power flow from the electrical machine and the engine through the EVT for each point, typically using engine speed and torque as two key criteria to determine the power flow from the primary power source and the hybrid transmission system. Determining these points along with the operator torque request solves the dynamic system equations and determines the power flow from the energy storage system. In this maneuver the engine speed changes to follow the optimal quasi-steady state operating point. It may accelerate to a high engine speed from idle and back down as the throttle input is reduced back to zero, with additional torque generated by energy transfer to the electric machine to the EVT. In the case of a throttle tip in to a steady state point, the engine reaches its optimal operating speed by following the optimal engine speed trajectory as defined by the current control system logic. In this system there are fixed ramp rates for engine speed changes. The fixed ramp rates are typically set as maximum control limits, and do not adjust for transient maneuvers. Solving the equations to meet the operator torque request in this manner does not optimize the system for transient operation.
What is needed is an optimization scheme for a hybrid powertrain system which looks at the combination of the power sources over a range of operating points that occur during a transient event, e.g., a vehicle acceleration event resulting from an operator torque request. It is desirable to develop an optimization scheme for vehicle operation which optimizes the system for transient operation and more fully utilizes the capability of the electrical energy storage system while ensuring the management and protection of the ESS under launch conditions, to meet the operator torque request.